The President was outlining his near-term agenda for joint work with Congress, one day after the Republicans gained control of both chambers in Tuesday's mid-term elections in a heavy blow to Obama and his fellow Democrats.

"I'm going to begin engaging Congress over a new authorization to use military force against ISIL," Obama said at a White House press conference, using another acronym for the IS.

"The world needs to know we are united behind this effort, and the men and women of our military deserve our clear and unified support," he added.

The White House is justifying ongoing US-led airstrikes on the IS targets inside Iraq and Syria with an Iraqi request for assistance and a 2001 congressional authorisation to fight terrorists.

Obama said the idea of new authorisation is "to rightsize and update whatever authorisation Congress provides to suit the current fight, rather than previous fights."

"It makes sense for us to make sure that the authorisation from Congress reflects what we perceive to be not just our strategy over the next two or three months, but our strategy going forward, " he added.

The President has vowed to "degrade and ultimately destroy" the Islamic State, a process he said would go well beyond his presidency that ends in January 2017.

The airstrikes joined by a number of countries have failed to stop the group's advances in Iraq and Syria and come under increasing attacks both at home and abroad.

Obama cautioned that "it's too early to say whether we are winning," calling the campaign "a long-term plan" aiming to solidify the Iraqi government and security forces and isolate the areas in Syria where the Islamic State can operate.

The militant group proclaimed a caliphate in late June in areas under its control in Syria and Iraq. "Our first focus here is to drive ISIL out of Iraq," Obama said.

"And what we are doing in Syria is, first and foremost, in service of reducing ISIL's capacity to resupply and send troops and then run back over the Syrian border, to eventually reestablish a border between Iraq and Syria so that slowly Iraq regains control of its security and its territory."